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A slower pace in summer does not, by itself, show that you have lost fitness. Heat and humidity make it harder to shed body heat while running, so the same pace can demand more effort than it does in cooler weather. Garmin puts it plainly: struggling to hold your usual pace during a heat wave “doesn’t necessarily mean your cardiorespiratory fitness (VO2 max) has dropped.”
Why is my running pace slower in the summer?
In hot, humid conditions, your body has to work harder to regulate temperature as well as keep you moving. That extra strain can make a familiar pace feel more difficult or lead you to slow down. Garmin describes heat and altitude as environmental factors that can affect performance and acclimation status on supported devices (Garmin’s heat and altitude acclimation explanation).
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That makes pace alone a poor comparison between a cool-weather run and a hot-weather one. The available evidence does not establish a universal seconds-per-mile adjustment for each degree of heat, so a fixed pace penalty is not a reliable way to judge your fitness.
Do these 3 things before closing this tab:
1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsDoes running slower in the heat mean I’m losing fitness?
No single hot-weather slowdown proves fitness loss. It may reflect the conditions rather than a change in your underlying cardiorespiratory fitness. But “doesn’t necessarily” is not the same as “never”: pace can be affected by other factors, too, and a watch estimate is not a diagnosis.
#1 Best Overall
- Easy-to-use running watch monitors heart rate (this is not a medical device) at the wrist and uses GPS to track how far, how fast and where you’ve run.Special Feature:Bluetooth.
- Battery life: up to 2 weeks in smartwatch mode; up to 20 hours in GPS mode
- Plan your race day strategy with the PacePro feature (not compatible with on-device courses), which offers GPS-based pace guidance for a selected course or distance
- Run your best with helpful training tools, including race time predictions and finish time estimates
- Track all the ways you move with built-in activity profiles for running, cycling, track run, virtual run, pool swim, Pilates, HIIT, breathwork and more
For a more useful comparison, look at how effort and performance change across runs in reasonably similar conditions. Consider the route and your training history as well as the weather. This is a practical way to make comparisons fairer, not a validated test that can isolate fitness from every other influence.
Does Garmin account for heat in VO2 max?
Garmin’s answer depends on the model, activity and data available. Its Forerunner 945 LTE Owner’s Manual, revision 6 (February 2024), says that heat-acclimation notifications and corrections to VO2 max and training status use a temperature threshold above 22°C (72°F). For that model, the feature is available only for GPS activities and requires weather data from a connected phone (Forerunner 945 LTE manual: Heat and Altitude Performance Acclimation).
Rank #2
- Easy-to-use running smartwatch with built-in GPS for pace/distance and wrist-based heart rate; brilliant AMOLED touchscreen display with traditional button controls; lightweight design in 43 mm size
- Up to 11 days of battery life in smartwatch mode and up to 19 hours in GPS mode
- Reach your goals with personalized daily suggested workouts that adapt based on performance and recovery; use Garmin Coach and race adaptive training plans to get workout suggestions for specific events
- 25+ built-in activity profiles include running, cycling, HIIT, strength and more
- As soon as you wake up, get your morning report with an overview of your sleep, recovery and training outlook alongside weather and HRV status (data presented is intended to be a close estimation of metrics tracked)
Those requirements are specific to the Forerunner 945 LTE manual; they should not be assumed to apply to every Garmin watch. Garmin’s VO2 max guidance explains the company’s running metric, while the acclimation feature provides environmental context on supported devices (Garmin’s VO2 max explanation). Neither should be treated as a direct diagnosis of whether your fitness has changed.
How long does it take to acclimate to running in the heat?
Heat acclimation develops through repeated exposure over days, not in a single run. In a 1998 review, K. B. Pandolf reported that roughly two-thirds to 75% of physiological adjustments and performance improvements may occur within four to six days, with nearly complete exercise-heat acclimation taking seven to 10 days. The review also notes variation in how acclimation is retained; it does not support a universal number of days after which it disappears (Pandolf, “Time course of heat acclimation and its decay,” PubMed).
Rank #3
- Easy-to-use running smartwatch with built-in GPS for pace/distance and wrist-based heart rate; brilliant AMOLED touchscreen display with traditional button controls; lightweight design in 43 mm size
- Up to 11 days of battery life in smartwatch mode and up to 19 hours in GPS mode
- Reach your goals with personalized daily suggested workouts that adapt based on performance and recovery; use Garmin Coach and race adaptive training plans to get workout suggestions for specific events
- 25+ built-in activity profiles include running, cycling, HIIT, strength and more
- As soon as you wake up, get your morning report with an overview of your sleep, recovery and training outlook alongside weather and HRV status (data presented is intended to be a close estimation of metrics tracked)
What one small running study found
A 2017 controlled study by James and colleagues assigned 17 participants to either five days of short-term heat acclimation (10 people) or work-matched control training (7 people). Testing compared cool conditions (13°C and 50% relative humidity) with hot conditions (32°C and 60% relative humidity). In the hot 5-km time trial, the acclimation group improved by an average of 6.2%, compared with 0.6% in controls; the reported difference between groups was statistically significant (p = 0.029) (James et al., 2017, PubMed).
This is evidence that acclimation helped performance in that study, not a promise that every runner will improve by 6.2% after five days. The sample was small and the protocol controlled; individual results can differ.
Quick Recap
Best Value
- Designed with a bright, colorful AMOLED display, get a more complete picture of your health, thanks to battery life of up to 11 days in smartwatch mode (5 days display always-on)
- Body Battery energy monitoring helps you understand when you’re charged up or need to rest, with even more personalized insights based on sleep, naps, stress levels, workouts and more (data presented is intended to be a close estimation of metrics tracked)
- Get a sleep score and personalized sleep coaching for how much sleep you need — and get tips on how to improve plus key metrics such as HRV status to better understand your health (data presented is intended to be a close estimation of metrics tracked)
- Find new ways to keep your body moving with more than 30 built-in indoor and GPS sports apps, including walking, running, cycling, HIIT, swimming, golf and more
- Wheelchair mode tracks pushes — rather than steps — and includes push and handcycle activities with preloaded workouts for strength, cardio, HIIT, Pilates and yoga, challenges specific to wheelchair users and more (data presented is intended to be a close estimation of metrics tracked)
Rank #4
- Brilliant AMOLED touchscreen display with traditional button controls; lightweight design in 46 mm size
- Up to 13 days of battery life in smartwatch mode and up to 20 hours in GPS mode
- As soon as you wake up, get your morning report with an overview of your sleep, recovery and training outlook alongside HRV status, training readiness and weather (data presented is intended to be a close estimation of metrics tracked)
- Plan race strategy with personalized daily suggested workouts based on the race and course that you input into the Garmin Connect app and then view the race widget on your watch; daily suggested workouts adapt after every run to match performance and recovery
- Training readiness score is based on sleep quality, recovery, training load and HRV status to determine if you’re primed to go hard and get the most out of your workout (data presented is intended to be a close estimation of metrics tracked)
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